The present invention relates to a method for operating an internal combustion engine with a variable compression ratio with the following steps: determining a theoretical value for the compression ratio; adjusting the compression ratio to the theoretical value; and correcting the adjusted compression ratio as a function of signals of a sensor mechanism of the combustion engine.
In addition, the present invention relates to a device for operating an internal combustion engine, which has at least one controller for adjusting a variable compression ratio to a theoretical value and a sensor mechanism, which is sensitive to changes of the compression ratio.
Such a method and device are known from DE 199 50 682 A1.
Methods for operating a multi-cylinder combustion engine with a variable compression ratio, as well as a control apparatus for controlling the method, are known, respectively, from DE 100 51 271.
These documents show a combustion engine, whose crankshaft is not supported directly in an engine block. Instead, the crankshaft is supported in eccentric rings, which are rotatably supported in support bearings in the engine block. With the aid of an adjustment mechanism, the eccentric rings can be rotated in a controlled manner. With rotation of the eccentric rings, the position of the crankshaft changes relative to the engine block. While the cylinders of the combustion engine are connected with the engine block, the pistons of the combustion engine moveably guided in the cylinders are connected via piston rods of constant length with the crankshaft. A change of the position of the crankshaft relative to the engine block results also in a change of the position of the pistons in the cylinders of the combustion engine. In particular, the position of the upper dead center (OT) of the pistons in the cylinders changes. As a result, the compression volume VK enclosed via the pistons in the upper dead center position also changes. Since the lower dead center position of the pistons change in the same manner as the upper dead center position, the displaced volume VH of the combustion engine does not change with a change of the crankshaft position relative to the engine block. The change of the compression volume VK with constant displaced volume VH implies a change of the compression ratio ε=(VH+VK)/VK.
Alternatively to this adjustment mechanism, in which the distance between the crankshaft bearing and cylinder is controllably changed, also systems are known, in which the compression ratio is changed by means of a tilting of the engine block relative to the crankshaft bearing, or by means of tilting the cylinder head relative to the engine block, or by means of raising or lower the cylinder head relative to the engine block in a control manner. All of these methods have in common that the geometric compression ratio ε=(VK+VH)/VK can be changed by means of a controlled changed of the compression volume VK.
In contrast to common combustion engines, which have a fixed compression ratio ε determined by means of the geometry of the combustion chamber, with a variable compression, the thermodynamic efficiency of the combustion engine can increase in partial-load operational range. As a result, consumption advantages can be achieved. Also, a reduction of the CO2 emission is connected with this. The higher the compression ratio, the higher the compression final temperature will be. Because with increasing compression temperature, the danger increases that knocking combustions occur, the maximum possible compression ratio is limited by the predisposition to knocking of the fuel that is used.
With common combustion engines with fixed compression ratios, the maximum compression ratio is constructively fixed, such that with maximum combustion chamber filling (full load), still no knocking occurs. As a result, with a constructively predetermined compression ratio and combustion chamber filling below a maximum possible value (partial load), critical compression temperatures are not reached easily. The efficiency of the combustion falls below an optimal efficiency. With the variable compression, this efficiency loss can be counteracted. Typically, the geometric compression ratio of a combustion engine is increased with variable compression with increasing load (combustion chamber filling).
With the above-noted DE 199 50 682 A1, a correction of the adjusted compression ratio takes place as a function of signals of a knock sensor of the combustion engine. From the actual angle of ignition, with which the knocking combustion occurs, the direct adjusted compression of the combustion engine should be closed. This teaching, however, does not take into consideration that the occurrence of the knocking combustion is dependent on further parameters, of which, for example, the fuel quality and the intake temperature should be noted. A reliable association of knocking combustions to a determined value of the compression, therefore, is not possible. In addition, for a reliable determination of the compression on the basis of signals of a knock sensor, multiple knocking combustions are necessary, which, based on noise comfort levels and mechanical performance of the combustion engine, is undesirable.
Based on the above concerns, an object of the present invention is to provide a method for adjusting the compression of a combustion engine with variable compression, which is not associated with the above disadvantages. In addition, the objection of the present invention is to provide a device for adjusting the compression which eliminates the above disadvantages.
This object is resolved according to the method of the present invention, which includes the step of correcting the step of determining an actual value of the compression ratio.
In addition, this object is resolved with a device of the above-mentioned type, which includes a control apparatus, which determines from the signals of the sensor mechanism an actual value of the compression ratio and controls the controller as a function of the actual value.